Process of reacting cellulose paper of low water content with gaseous formaldehyde



Mardl 21, 1967 J. RUSSELL ETAL 3,310,363

PROCESS OF REACTING CELLULOSE PAPER OF LOW WATER CONTENT WITH GASEOUS FORMALDEHYDE Filed May 24, 1963 Haw 715/2 DE) HCf/U GAS 1'71, ODISCHfl/EGE INVENTORS. div/ 155 PUSSELL.

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3,310,363 PROCESS OF REACTING CELLULOSE PAPER F LOW WATER CONTENT WITH GASEOUS FORMALDEHYDE James Russell and Willard E. Carlson, New City, N.Y., and Charles H. Mayhood, Jr., Allendale, N.J., assignors to St. Regis Paper Company, New York, N.Y., a corporation of New York Filed May 24, 1963, Ser. No. 282,931 Claims. (Cl. 8116.4)

This invention relates to a method for improving the wet strength and particularly the wet stiffness or rigidity of a paper article. It also relates to a method for reducing the water vapor permeability of paper. The invention is particularly applicable to a method for improving the wet rigidity and wet strength of corrugated paperboard, especially a scored and slotted, knocked down corr-ugated paperboard box blank. One embodiment of the present invention is directed to a novel corrugated paperboard product of high wet strength and wet rigidity constructed with a normally water dispersible glue, but which adhesive has been rendered water insoluble by treatment at the side of its application and used in the corrugated paperboard.

Previous attempts to reduce the water sensitivity of paper have involved the addition of wet strength resins to the pulp at the wet end of the paper making process, or the saturation of an already produced paper product with a wet strength-imparting material. Neither of these techniques has been entirely satisfactory. Post saturation of a corrugating medium with a water solution of formaldehyde has the drawback that the treated material having the required wet rigidity is enitrely too brittle to enable converting to corrugated board. Wet end additions of such well known wet strength materials as urea formaldehyde or melamine formaldehyde produce a paper sheet or corrugating medium, for example, with substantially improved wet strength, but these materials do ,not materially increase the wet stiffness of the paper; that is to say, they produce wet stiffness only up to about 10-15% of the dry stiffness.

The present process involves the treatment of paper articles, including flexible and stiff sheets as well as corrugated paperboard with formaldehyde gas which reacts with the cellulose of the paper apparently cross-linking the same, but definitely reducing the sensitivity of the paper to water. Paper has been treated heretofore with aqueous formaldehyde and an acid catalyst and also with formaldehyde gas, but the results obtained by these processes have not been satisfactory because the paper or corrugated board has been unduly embrittled by the methods employed.

Formaldehyde gas as employed herein shall be understood to mean monomeric formaldehyde vapor. It can be produced by heating polymeric formaldehydes or hemiacetals, for example.

It is a primary object of the present invention to provide a novel method for the treatment of paper and corrugated paperboard with formaldehyde gas in such a manner that wet rigidity and wet tensile strength are ,very substantial-1y raised, even about as much as about method for producing a corrugated paperboard article,

for example a box or box blank, which remains satisfactorily rigid when wet.

It is a still further object of the present invention to provide a continuous process for the improvement of paper or corrugated paperboard to render the same relatively water insensitive, which involves only a few seconds treatment.

' United States Patent 0 F 3,310,363 Patented Mar. 21, 1967 It is another object of the present invention to provide a method for greatly reducing the water vapor per meability of paper without unduly embrittling the same.

It is still another object of the present invention to provide a corrugated paperboard box blank of improved wet rigidity which is constructed with the use of a glue or adhesive which is rendered water insoluble by the gas treatment which is employed to reduce the water sensitivity of the corrugated medium and liner sheets of the article.

In accordance with the process of this invention paper, which term shall be understood to include corrugated paperboard, is contacted with formaldehyde gas at elevated temperature and at a comparatively very low moisture content of the paper article as compared with its moisture content at ambient atmospheric conditions, depending upon the reaction temperature, in the presence of an acid catalyst in contact with the paper for the reaction between formaldehyde and cellulose.

A catalyst is required for the reaction between formaldehyde vapor and the paper, and a number of catalysts may be satisfactorily employed. The catalyst is preferably acidic, for example, a mineral acid, and desirably the vapor of a mineral acid. The presently preferred catalyst is dry hydrogen chloride, and it may be applied to the paper either before or after treatment with formaldehyde gas or it may be applied in admixture with the formaldehyde gas.

Alternatively, the catalyst may be a material which at the temperature of reaction dissociates or reacts with the paper to produce hydrogen chloride in quantities sufficient to catalyze the reaction between formaldehyde and the paper. For example, an acid salt such as ammonium chloride may be added to the paper stock so that the sheet or board produced will contain a small quantity of solid ammonium chloride dispersed therethrough. At the elevated temperatures of the present process and at the moisture content of the paper being treated, though it be small, some dissociation of ammonium chloride occurs to release hydrogen chloride in amount sufficient to catalyze the reaction. Of those materials which release the required hydrogen chloride upon reaction with the paper, we particularly point to the chlorosilanes, especially dimethyldichlorosilane. Thus, while the hydrogen chloride is desirably imparted to the paper as a gas, it may be present on the paper as a constituent or dissociation product of a material which has been previously applied to the paper or dispersed therethrough.

Chlorine gas has also been found to be a very effective catalyst, producing excellent results when present in amounts of as little as 1% by weight of the paper being treated with formaldehyde gas. One drawback to chlorine is its bleaching action upon the paper being treated. We have investigated other catalysts, including organic acids such as formic and acetic, and the results obtained were satisfactory, but reaction time even at temperatures of C. and above was unduly long as compared with chlorine and hydrogen chloride catalysts. Sulfur dioxide was also tested as a catalyst and found to be satisfactory, but again unduly long reaction times were required, for example of the order of 30 minutes at 150 C. as compared with less than 60 seconds for hydrogen chloride. With sulfur dioxide, the required catalyst concentration is quite high, being of the order of about 20% as compared with less than 1% by weight of the paper in the case of hydrogen chloride.

As noted above when hydrogen chloride gas is employed the paper may be contacted with the catalyst before or after treatment with formaldehyde, but most desirably the paper is treated with the mixed gases formaldehyde and hydrogen chloride. In so doing only one gas contact is required, and hydrogen chloride is not in contact with the paper for too long a time, which is paper product.

often the case where two steps are employed with hydrogen chloride preceding application of formaldehyde gas to the paper. It is important in accordance with the present process that the moisture content of the paper be controlled to below 2%, and preferably below 1%, and that the paper is not in contact with hydrogen chloride for any longer time than necessary to catalyze the reaction. The acid has an embrittling effect upon the paper.

Throughout this application we refer to moisture content of the paper below about 2%. By this we mean water which may be removed by simple heating of the paper, that is to say absorbed moisture and not water of hydration or water chemically bound to the cellulose and not normally removed by non-destructive heating, for example.

It is our discovery of the critically important role of moisture in the paper at the time of treatment with formaldehyde gas and the acid catalyst which constitutes what we believe to be the most significant advance in the art and the most important feature of this invention. The embrittlement which our process avoids is of course a relative term depending upon the end use of the treated For example, the liner sheets of corrugated board must not be so brittle that they cannot be scored and/or bent without cracking, Whereas papers not subjected to folding, e.g., dimensionally stabilized card stock, may be relatively more brittle and yet entirely satisfactory.

It is relatively easy to impart wet stiffness and wet strength to corrugated paperboard, for example, simply by treating the board with formaldehyde gas in the presence of an atmosphere containing hydrogen chloride, but where the contact of the paper with this acid vapor is prolonged and the moisture content of the paper being treated is above about 2% by weight of the paper, very significant embrittlement of the paper occurs. case of corrugated board the brittleness so produced causes breaking of the liners when the board is bent or scored in severe cases, and produces very weak scored areas in less severe cases. The brittleness which the present process avoids is of course made manifest while the board or paper is dry. The degree of brittleness imparted to the paper after treatment may be gauged from the changes in dry stretch, modulus and tensile strength of the material.

In the present process the reaction of formaldehyde gas with the paper is carried out at elevated temperatures. These temperatures may range from about 70 to a maximum of about 180 C., with these figures referring to the actual temperature of the paper or board being treated. Certain papers will of course char above about 160 C. and of course such materials will not be subjected to temperatures which will cause charring. Corrugated board, on the other hand, can accommodate temperatures as high as 180 C. for short periods without ill effects. In a preferred operation of the present process corrugated paperboard is at a temperature between about 120 and 150 C. when contacted with the formaldehyde and catalyst. The formaldehyde and hydrogen chloride or chlorine gas may of course be at substantially higher temperatures, for example even as high as 250 C. or above, provided of course that contact time with the paper is so limited that the paper does not exceed about the upper limit of the above specified range.

In order to treat the paper as rapidly as possible its temperature at the time of contact with formaldehyde and catalyst will be as high as possible, consistent with limitations of catalyst concentration and moisture content of the paper, to produce the desired wet strength and wet rigidity improvement while also avoiding embrittlement of the material. Within the range 70 to 180 C. the reaction between formaldehyde and the paper occurs most rapidly at the upper end of this range, but at this high temperature, contact time must be exceedingly short to avoid embrittlement, that is to say of the order of a half In the minute or less. The situation is complicated by the fact that at a given temperature and contact time, embrittlement increases as moisture content of the paper increases up to the permissible maximum of 2%. As a practical mater some moisture is required to render the catalyst effective since apparently some ionization of the catalyst is involved in its successful facilitation of the formal dehyde-cellulose reaction in and on the paper.

The moisture content of corrugated paperboard, for example at ambient conditions is at least about 56% of the weight of the board, depending of course upon the season and the humidity. At 72 F. and 50% R.H. most corrugated boards contain about 6 to 8 /2% by weight water, and paper in sheet form contains slightly less.

Accordingly, the first step in the process is reduction of the moisture content of the paper. Since it has been found that the moisture content of the material being treated is directly related to embrittlement of the product, which in turn is related to reaction temperature, contact time and the quantity of the acid catalyst, the paper is reduced in moisture content to a sufficiently low level as to preclude production of an unsatisfactorily brittle product. As a practical matter, 2% moisture can be tolerated when the temperature of the paper during reaction with formaldehyde gas and contact with HCl gas is not above about 70 C. At the other end of the temperature range the moitsure content of the paper should be no higher than about 0.2%. As a practical matter, corrugating medium and corrugated paperboard should be reduced to less than 0.5% moisture prior to contact with the formaldehyde and acid catalyst at -150 C. Reduction of the moisture content of the paper or board may be to as low as 0.1% by weight or even less, say to the order of 0.05% moisture by weight.

Dehydration may be accomplished by heating the paper in a relatively dry atmosphere in an oven or by subjecting the paperboard, for example, to a stream of hot dry air which is directed through the board lengthwise of the corrugating flutes. On the other hand, the moisture content of the board may be reduced by subjecting the same to substantially reduced air pressure and a desiccant With or without heating. Since the board at the time of reaction must be at an elevated temperature, preliminary heating to say 120-150" C. is the most convenient method of reducing the moisture content of the board and at the same time raising the board temperature.

'Immediately following heating, the paper or board in a preferred embodiment of the present method is subjected in a confined zone to a heated gas stream containing formaldehyde and hydrogen chloride and contact is maintained for the necessary time, depending upon temperature, during which the reaction between formaldehyde and the paper occurs. Alternatively, the paper may be first treated with dry formaldehyde gas and then with dry hydrogen chloride, or the procedure may be reversed. Chlorine may of course be employed instead of hydrogen chloride, or a heated board which contains solid ammonium chloride in small quantity may be simply subjected to the formaldehyde gas stream. The gas stream referred to may of course be diluted with an inert gas, for example air.

In the presently preferred method the reaction is carried out by introducing paperboard or paper sheet in open width into a stream of moving gas. However, satisfactory results can also be obtained by bleeding dry formaldehyde gas and dry hydrogen chloride simultaneously or sequentially into an oven in which paperboard has been stacked. The former method is described in further detail hereinafter and is employed in the continuous processing of paper sheet material and corrugated board.

As a practical matte-r it is not necessary to carry out the reaction between the paper and formaldehyde at reduced pressure. Our initial work on this process was conducted in an oven at a vacuum pumped to the extent of about 10 to 12 inches of mercury primarily to enable introduction of formaldehyde gas and the catalyst gas.

However, the process has also been carried out in a confined zone at substantially atmospheric pressure with equally good results, or at elevated pressure. When contactbetween the formaldehyde and catalyst gases and the paper is accomplished by passing the paper article through a moving stream of these gases the pressure in the stream is desirably at or just slightly above atmospheric. Thus it presently appears that some of the advantages which would seem to accrue with the use of elevated or re duced pressure, e.g., rapid penetration of the paper by the gas, it not necessary in order to achieve an entirely satisfactory treated paper.

As the process is presently envisioned for continuous operation on a commercial scale for the treatment of corof the paper and immediately followed by contact with the indicated quantity of the catalyst. The reaction was carried out at a temperature of 150 C. for the reaction times indicated. The physical properties measured were machine direction tensile, stretch and modulus, both wet and dry, all of which were determined on an Instron tester employing 1 inch strips. The dry samples were conditioned for twelve hours at 72 F. and 50% RH. and the wet properties were determined on samples which had been soaked in water at 65-70 F. for the same period of time. As is well known, the modulus is the initial slope of the stress-strain diagram produced on the Instron tester. Technically, it is a measure of the resistance of the paper to stretch. With this paper sheet a rugated paperboard, for example in the form of cut and dry stretch of about 1.2% or less was indicative of a scored box blanks, the reaction temperature, that is to sample which was too brittle because it could not he say the temperature of the board at reaction will be befolded without cracking. The wet modulus is the most tween about 120 and 150 C. With a minimum of important measurement for determining the wet stiffness about 1% by weight formaldehyde gas coming in conproperties of the paper. In each of the above tests the tact with the board and catalyst present in amounts from moisture content of the paper just prior to and during the about 0.5 to 50% by weight of the formaldehyde in the reaction was below about 2% by weight.

TABLE I Dry Properties Wet Properties Quantity of Reaction Catalyst Catalyst Time (Percent of (minutes) Tensile Stretch Modulus Tensile Stretch Modulus Paper) (1h (percent) (lbs./ (lbs) (percent) (lbs./

percent) percent) 45. 0 1. 8 49. 3 1. 3 0. 9 1. 6 46.7 1.5 49.3 17.7 2.6 10.4 46.7 1.5 48.3 16.6 2.4 10.3 47.1 1.6 51.9 17.4 2.7 10.0 44. 9 1. 7 44. 3 18.6 2. 5 11.6 42.2 1.3 51.0 17.5 1.9 14.0 44.5 1.6 46.6 17.7 2.7 9.7 46.5 2.2 44.3 7.0 2.0 5.0 NHtol 43. 5 1. 6 41. 5 17.6 2. 9 9. 4 Dimethyldichlorosilane 49. 7 1. 6 49. 2 18. 5 2. 7 11. 6

case of hydrogen chloride, for example, reaction times are less than seconds and can be as low as even 1 or 2 seconds provided the partial pressure of formaldehyde in the gas stream is quite high in the gas mixture and the reaction temperature is 150 C. or higher within the aforementioned broad range. Generally, however, at 120- 150 C. contact times will be between about 3 and 15 seconds. A mixed dry formaldehyde and hydrogen chloride gas is preferably employed, and the temperature of about 120 C. should be exceeded in order to dissociate the formaldehyde polymer which forms in the presence of hydrogen chloride. This dissociation is desired not only to facilitate the reaction, but also to reduce the water soluble formaldehyde content of the treated paper product to an acceptable low level.

The effects of various catalysts upon the reaction between formaldehyde gas and a corrugated board component in sheet form (33 lbs/1000 ft?) are set forth in the table below. In each test the sheet was contacted From the above it will be immediately apparent that hydrogen chloride, chlorine and ammonium chloride are outsandingly satisfactory catalysts.

The effect upon the product of the moisture content of the paper during treatment is dependent upon temperature, contact time, and upon the acidity of the catalyst and the catalyst concentration. This is illustrated by a series of tests conducted with the same 33 lb. paper at reaction temperatures of and C. are set forth in the following table. The paper was contacted with 2% by weight formaldehyde gas and about /2% by weight of hydrogen chloride. Contact time was 1 minute, and the reaction was carried out in an oven. Water in the reported percents by weight of the paper was introduced to the oven and allowed to vaporize prior to introduction of the formaldehyde and hydrogen chloride gases. The water absorbed, reported as moisture content, was determined by weighing the original oven dry paper and the paper after the specified quantity of water had been with formaldehyde gas in amounts of about 5% by Weight 40 added to the oven.

TABLE II Dry Properties Wet Properties Added Moisture Temperature, C. Moisture, Content,

' percent percent Tensile, Stretch, Modulus, Tensile, Stretch, Modulus,

lbs. percent lbs./perlbs. percent lbs/percent cent The most significant conclusion which can be drawn from Table II is the fact that in order to obtain paper with good wet and dry tensile strengths and wet rigidity the temperature must be kept high and the moisture content low. For example, at a temperature of 150 C. the paper should contain less than 0.5% moisture. As noted in connection with Table I, with this paper the edge of brittleness is reached at a dry stretch of about 1.2%. That is to say, the treated paper which has a dry stretch of 1.2% or less was brittle and was not commercially acceptable for the preparation of folded corrugated paperboard.

Still referring to Table II it will be observed that acceptable properties can be obtained at 90 C. when the moisture content is about 1.8%. However, the dry properties of the paper are poor, with the low stretch and high modulus indicating that the material is definitely brittle. Table II definitely indicates that there is an interaction between water in the paper and the reaction temperature, at a given catalyst concentration, which affects brittleness. At 150 C. the corrugating paper containing 0.5% moisture was embrittled whereas the same paper with 0.2% moisture was not. At 120 C. the paper with 1.3% moisture was embrittled, while the sample containing only 0.7% was not. At 90 C. the paper with 1.8% moisture was brittle while the one with 1.25% was not.

The process of this invention may be carried out in a continuous manner or batchwise. The batch process of course requires a large oven or treating room and the corcurgated paperboard, box blanks or paper sheets must be so arranged that adequate gas contact can be achieved. Preferably, the process is carried out continuously. That is to say, the paper in sheet form and in open width, or corrugated paper-board, is passed into a zone which is maintained at a substantially constant formaldehyde and catalyst gas concentration and water vapor content. A typical continuous system is outlined in the accompanying drawing, wherein FIG. 1 is a flow diagram of the process which includes a treating zone illustrated in section and,

FIG. 2 is sectional elevation of the treating zone taken on the lines 22 of FIG. 1.

Referring now to the drawings, there is provided a treating zone 11 in the closed system 20,- which may be an enlarged section of pipe or conduit which is slit as at 12 and 13 thus dividing zone 11 into structural halves 14 and 15 as shown in FIG. 2. The material to be treated corrugated paperboard 10, for example, is passed in continuous fashion through treating zone 11 in the direction indicated by the arrow in FIG. 2. Suitable flexible seals 16 and 17 prevent escape of the treating gas from zone 11. Movement of the gas stream is through the corrugated board in a direction lengthwise of the corrugated flutes 5. Before introduction to the treating zone the corrugated board or paper sheet, for example, is suitably dehydrated as indicated above, preferably by treatment for the required time in an oven or in a moving hot air stream so that when it enters the formaldehyde treating zone it is at or substantially at the desired reaction temperature.

Dry formaldehyde and dry hydrogen chloride gases are present in the closed system 20' and are drawn through the treating zone and recirculated by blower P. A suitable heater is provided in the system to maintain the gases at a desired temperature above that of the paperboard, and suitable thermostatic controls are provided so that the board maintains the desired reaction temperature in the treating zone. The gas stream circulating through the treating zone may be diluted with dry air, in which event the formaldehyde content of the gas will preferably range between about and 85% by weight of the gas. The catalyst, hydrogen chloride, under these conditions will preferably be present in the system in amounts of about 0 .5 to 50% by weight of the formaldehyde. The system may be operated without an added diluent gas, in which case the gas stream is essentially only formaldehyde and catalyst. Of course air will enter the system in the board and water vapor will be formed by the reaction. It will of course be appreciated that the speed of travel of the corrugated board or paper sheet through the treating zone and also the speed of circulation of the stream of treating gas and the composition of these gases will determine the reaction time. Commercial realization of the present process dictates as short a contact time as possible to produce the desired maximum wet properties.

Water is produced by the condensation of formaldehyde with the cellulose of the paper, and must be removed from the closed system lest the paper absorb too much moisture while in the treating zone. Accordingly, a suitable gas dehydrating unit is provided in the system 20, for example in shunt line 21, for metering and removal of moisture from the circulating gas stream. Suitable sensing devices, not shown in the diagram, determine temperature of the circulating gas and its content of formaldehyde and hydrogen chloride. These sensors may be employed to operate the valves, V, to in troduce formaldehyde as it is consumed, and to introduce catalyst which is either removed with the board or during water vapor removal from the system.

Treatment of paper in accordance with the present process very substantially improves the wet strength and stiffness of the material, and also improves the effectiveness of the paper as a water vapor barrier. In other words, treatment in accordance with the present invention reduces the water vapor permeability of the paper.

Example 1 A typical dense paper coil insulating kraft of a caliper of 0.00 25 inch was treated in accordance with the present process. The paper was predried at C. and then subjected in a vacuum oven to 5% by Weight formaldehyde gas and an HCl gas catalyst :both at 150 C. for one minute.

The paper sheet was then tested for water vapor permeability before and after treatment by disposing the sheet in a testing device where it separated a dry atmos phere containing calcium chloride from air which was maintained at 72 F. and 50% RH. The quantities of moisture passing through the sheet after the specified time are set forth in the following table.

These result are quite surprising and show that the formaldehyde definitely reacts with the cellulose molecules of the paper. Wet strength resins, on the other hand, do not affect water vapor permeability. Accordingly, the reaction involved here is not the same as that involved with wet strength resins. Also, the wet properties of the paper so treated, and particularly the wet modulus, are a substantially higher percent of the dry properties of the same paper than is the case with wet strength resin-treated papers.

In many instances the paperboard treated in accordance with the present process will have been scored and slotted. The slotting and cutting operations produce a very substantial quantity of cut-out waste, referred to in the art as broke. In conventional kraft paper converting operations the broke is returned to a paper mill and blended with other pulp constituents. However, in the case of corrugated board which has been formed with a weather or water proof corru'gating adhesive which becomes water insoluble upon drying or which 9 upon drying is not again readily dispersible in water, this broke because of its water insoluble portion is difficult to completely repulp and so is of very substantially reduced value.

Example 2 With this invention, it is now possible to employ an A e i of te ts Were conduc ed on 9" x 11" panels adhesive between corrugated medium and liner sheets of 4? lb.36 lb.42 lb. corrugated board at temperatures which is water. dispersible before and after drying, but varying from 185 to 250 F. The panels were treated m which is rendered insoluble in water by the formaldehyde groups of 16 and 32 in a heated vessel capable of operatgas treatment employed in accordance with the present g at elevated Or r d Pr ss r he oard was invention to impart wet strength and Wet rigidity to the predried as indicated in the following table to well below box blank. Thus Where the present process is carried 2% by welght water. The formaldehyde was introduced out on slotted and/ or scored corrugated box plants the t0 the vessel under Va Pressure was adlusted Wlth adhesive in the broke will not preclude repulping. compressed arr and then HC1 gas was admitted. Re-

In accordance with the present invention the adheaction was for the times indicated. Pressure, temperasive employedto bond the corrugated edi m to the ture, and catalyst and formaldehyde concentrations were liner sheets contains as a minor constituents a polyhyvaried as shown. The treated samples were then soaked droxy polymer, a good example of which is polyvinyl in water at 657( F. for one hour, whereupon short alcohol, which when contacted with the formaldehyde column compression tests were conducted. Th s test gas at the aforesaid elevated temperatures is rendered involves subjecting a 1 x 4 1n. sample (the long dimension insoluble by cross-linking either withitself and/or with transverse of the corrugated flutes) to vertical compresthe cellulose of the paper. Polyvinyl alcohol is a consion on an Instron tester, and measuring the pound per stituent of many water insoluble adhesive compositions inch of width at its maximum loading. The conditions and is employed torender the adhesive water dispersiand results of the tests were as follows:

TABLE IV Treatment Reaction 7 Median Wt. Board, HOHO HCl Wet Column Sample No. g. Total Compression Time Temp. Pressure (#/in.) Partial Percent Percent Percent (min) I (in. Hg Pressure, onBoard onBoard on Absolute) Percent Wt. Wt. HCHO Untreated Board 2. 9 1 55 1. 2 0. 7 58 5 235-248 28. 2 12. 4 55 1. 2 0. 7 58 5 223-225 23. 5 12. 4 3. 5 1. 3 5 212-225 80. 0 11. 4 33 3.6 o. 9 25 5 235 50. 0 13. 0 3. 2 0. 6 19 5 235-245 40. 0 13. 2 66 3.4 '1. 0 29 5 2 190-230 30. 0 10. 5 66 3.4 1.0 29 2.5 200-220 30.0 9.4 66 3.4 0.8 24 0.5 210-220 29.0 10.0 (o) 53 37 1.7 0.8 47 1.0 185-230 30.0 9.4 (0) 1,400.... 15 1.0 0.19 19 5 212 80.0 4.5

1 Liquid water present in the treating zone.

2 Temperature varied below formaldehyde condensation temperature.

(C) Board Predried in Oven at 224 F. for 30 min.

ble. A typical such adhesive is polyvinyl acetate. Accordingly, polyvinyl alcohol is either added to the acetate prior to polymerization or to the polyvinyl acetates or formed in the production of the acetate polymer, in amounts just sutficient to render the polyvinyl acetate film water dispersible. The polyvinyl acetate adhesive may of course contain other ingredients to enable proper application to the paper,or in fact the complete material may contain additional adhesives such as starch. Upon drying of such adhesives the active adhering ingredients are polyvinyl acetate and any other materials such as starch which may be present. However, when immersed in water the polyvinyl alcohol serves its original function and renders the adhesive Water redispersible.

By the present process, however, such adhesives are rendered completely water insoluble by reason of the reaction of the polyvinyl alcohol component, which is only a minor constituent of most such adhesives, in contact with the paper with formaldehyde gas. This reaction destroys the ability of the polyvinyl alcohol to dissolve in water and accordingly, also the ability of this normally water soluble material to break up and redisperse the polyvinyl acetate, starch or other insoluble films making up the adhesive.

A preferred product in accordance with the present invention thus comprises a corrugated paperboard formed with the use of a normally water dispersible adhesive containing polyvinyl alcohol as the dispersing agent, which adhesive is rendered water insoluble by reason of the From the above it can be seen that effective treatments can be imparted over a wide range of pressure and temperature. Between and C. temperature has little effect. Partial pressure of formaldehyde appeared to influence treatment, but the effect leveled off at partial pressures above about 33% HCHO in the mixed gas.

Example 3 TABLE V Formaldehyde HCl Reaction Wet Column (liters/min.) (liters/ time Compression min.) (seconds) (lbs/in.)

Untreated board 2. 9

The above results substantiate the practicability of continuous treatment of corrugated board for very short times. Wet column compression results are somewhat lower than in Example 2 because the liner sheets were only treated incidentally on their interiors adjacent the corrugated medium. The boards of this example were formed with the polyvinyl alcohol-containing polyvinyl acetate adhesive, and following treatment the adhesive was not redispersible in water.

We claim:

1. A process for improving the wet rigidity of cellulose paper which comprises contacting said paper with at least 1% by weight formaldehyde gas based on the paper, and from 0.5 to 50% by weight hydrogen chloride gas, based on the weight of the formaldehyde, while maintaining the temperature of the paper at from about 70 C. to 180 C. during a contact time of from about 1 second to minutes at a water content in the paper between about 0.05% and 2% by weight based on the weight of the paper, the conditions of temperature, time and water content being so selected within the defined ranges so that as the temperature increases in the temperature range, the contact time decreases in the time range, and as the water content increases in the water content range, the temperature decreases in the temperature range, thereby insuring the least amount of embrittlement in the dry state consistent with improved wet rigidity.

2. A process as in claim 1 wherein the temperature is from 120 C. to 150 C. and the contact time is between 3 and seconds.

3. A process as in claim 1 wherein the paper is contacted with the formaldehyde gas and hydrogen chloride gas simultaneously.

4. A process as in claim 1 wherein the paper is contacted with the hydrogen chloride gas before it is contacted with the formaldehyde gas.

5. A process as in claim 1 wherein the paper is contacted with the hydrogen chloride gas after it is contacted with the formaldehyde gas.

6. A process as in claim 1 wherein the water content of the paper is reduced to from about 0.05 to 2% by weight before contact with the formaldehyde and hydrogen chloride gas within the defined conditions of temperature and contact time.

7. A process as in claim 1 wherein the paper is paperboard.

8. A process as in claim 1 wherein the paper is corrugated paperboard.

9. A continuous process for treating corrugated cellulose paperboard to improve its wet rigidity, which comprises heating the board to reduce its Water content to between about 0.05% and 2% by weight based on the weight of the paperboard, and maintaining said paperboard water content during the process, providing a moving stream of gases comprising at least 1% by weight formaldehyde gas based on the paperboard and from 0.5% to 50% by weight of hydrogen chloride gas, based on the weight of the formaldehyde, at a temperature from about 70 C. to 180 C., introducing said stream to a confined treating zone, so passing the heated, dried board through said treating zone that the stream of gas therein passes through the board in a direction lengthwise of the corrugating flutes, the contact time between said board and said stream being from about 1 second to 5 minutes, exhausting gas containing unreacted formaldehyde and hydrogen chloride from the treating zone, removing a portion of the water vapor produced by the reaction of formaldehyde with the paperboard from the exhausted gas and introducing the so dehydrated gas to the treating zone; the conditions of temperature, time and water content being so selected within the defined ranges that as the temperature increases in the temperature range, the contact time decreases in the time range, and as the water content increases in the water content range, the temperature decreases in the temperature range, thereby insuring the least amount of embrittlement in the dry state consistent with improved wet rigidity.

10. A continuous process for treating corrugated cellulose paperboard to improve its wet rigidity, which comprises heating the board to reduce its water content to between about 0.05% and 2% by weight, based on the weight of the paperboard, and maintaining said paperboard water content during the process providing a mov ing stream of gases comprising at least 1% by weight formaldehyde gas based on the paperboard and from 0.5 to 50% by weight of hydrogen chloride gas, based on the weight of the formaldehyde, at a temperature from about C. to C.,-introducing said stream to a confined treating zone, so passing the heated, dried board through said treating zone that the stream of gas therein passes through the board in a direction lengthwise of the corrugating flutes, the contact time between said board and said stream being from about 1 second to 5 minutes, exhausting gas containing unreacted formalde hyde and hydrogen chloride from the treating zone, removing a portion of the water vapor produced by the reaction of formaldehyde with the paperboard from the exhausted gas, adding make-up formaldehyde to the gas, and returning the so treated gas to the treating zone, while introducing fresh, dried board having the above recited water content to said zone; the conditions of temperature, time and water content being so selected within the defined ranges that as the temperature increases in the temperature range, the contact time decreases in the time range, and as the water content increases in the water content range, the temperature decreases in the temperature range, thereby insuring the least amount of embrittlement in the dry state consistent with improved wet rigidity.

References Cited by the Examiner UNITED STATES PATENTS 2,806,787 9/1957 Toulmin 8116.4 X 3,090,699 5/1963 Bulson 162-175 X 3,154,373 10/1964 Guthrie 8116.4

OTHER REFERENCES Arceneux et al.: American Dyestuff Reporter, July 23, 1962, pages 45-52.

Cohen et al.: Tappi, December 1959, pages 934-940.

Stamm: Tappi, January 1959, pages 4450.

NORMAN G. TORCHIN, Primary Examiner. H. WOLMAN, Assistant Examiner, 

1. A PROCESS FOR IMPROVING THE WET RIGIDITY OF CELLULOSE PAPER WHICH COMPRISES CONTACTING SAID PAPER WITH AT LEAST 1% BY WEIGHT FORMALDEHYDE GAS BASED ON THE PAPER, AND FROM 0.5 TO 50% BY WEIGHT HYDROGEN CHLORIDE GAS, BASED ON THE WEIGHT OF THE FORMALDEHYDE, WHILE MAINTAINING THE TEMPERATURE OF THE PAPER AT FROM ABOUT 70*C. TO 180*C., DURING A CONTACT TIME OF FROM ABOUT 1 SECOND TO 5 MINUTES AT A WATER CONTENT IN THE PAPER BETWEEN ABOUT 0.05% AND 2% BY WEIGHT BASED ON THE WEIGHT OF THE PAPER, THE CONDITIONS OF TEMPERATURE, TIME AND WATER CONTENT BEING SO SELECTED WITHIN THE DEFINED RANGES SO THAT AS THE TEMPERATURE INCREASES IN THE TEMPERATURE RANGE, THE CONTACT TIME DECREASES IN THE TIME RANGE, AND AS THE WATER CONTENT INCREASES IN THE WATER CONTENT RANGE, THE TEMPERATURE DECREASES IN THE TEMPERATURE RANGE, THEREBY INSURING THE LEAST AMOUNT OF EMBRITTLEMENT IN THE DRY STATE CONSISTENT WITH IMPROVED WET RIGIDITY. 